Characterization of secondary aerosol from the photooxidation of toluene in the presence of NOx and 1-propene

Abstract

Secondary organic aerosol (SOA) from the photooxidation of toluene in a hydrocarbon-NOx mixture was generated in a 190 m3 outdoor Teflon chamber. The photooxidation reaction of toluene in the gas phase leads to substituted aromatics (TOL-AR), nonaromatic ring retaining (TOL-R), and ring opening products (TOL-RO). In this work, the following ring opening oxycarboxylic acids were newly identified: glyoxylic acid, methylglyoxylic acid, 4-oxo-2-butenoic acid, oxo-C5-alkenoic acids, dioxopentenoic acids, oxo-C7-alkadienoic acids, dioxo-C6-alkenoic acids, hydroxydioxo C7-alkenoic acids, and hydroxytrioxo-C6-alkanoic acids. The newly characterized TOL-R and TOL-RO products included methylcyclohexenetriones, hydroxymethylcyclohexentriones, 2-hydroxy-3-penten-1,5-dial, hydroxyoxo-C6-alkenals, hydroxy-C5-triones, hydroxydioxo-C7-alkenals, and hydroxyC6-tetranones. Products in both the gas and aerosol phases were derivatized with O-'(2,3,4,5,6-pentafluorobenzyl)-hydroxylamine hydrochloride (PFBHA) for carbonyls and pentafluorobenzyl bromide (PFBBr) for carboxylic acid and phenol groups and analyzed using a gas chromatograph/ mass spectrometry (GC/MS) in an electron impact mode (EI) and a gas chromatograph/ion trap mass spectrometry (GC/ITMS) in both chemical impact and FI modes. To confirm different isomers, the PFBHA-derivatives of products were rederivatized by silylation using N,O-bis(trimethylsilyl)-trifluoroacetamide (BSTFA). The Fourier transform infrared spectroscope (FTIR) was used to obtain additional functional group information for SOA products impacted on a zinc selenide FTIR disk. The major SOA products under the high NOx conditions of the above experiment included methylnitrophenols, methyldinitrophenols, methylbenzoquinones, methylcyclohexenetriones, 4-oxo-2-butenoic acid, oxo-C5-alkenoic acids, hydroxy-C3-diones, hydroxyoxo-C5-alkenals, hydroxyoxo-C6-alkenals, and hydroxydioxo-C7-alkenals. Of the major SOA products, the experimental partitioning coefficients (iKp) of aldehyde products were much higher and deviated more from predicted iKp values. This is an extremely important result, because it shows that aldehyde products can further react through heterogeneous processes, which may be a very significant SOA generation mechanism from the oxidation of aromatics in the atmosphere.